Fire doors are generally made for the purpose of stopping or delaying the transfer of thermal energy (i.e., heat), from one side of the door to the other side. Current fire-resistant doors generally contain a fire-resistant core usually encased in a door-shaped shell, wherein the shell is made from various materials generally known to those of ordinary skill in the art. The core is customarily bonded or glued to both inside surfaces of the shell.
Fire doors, as used in residential, commercial, and industrial applications, typically are employed in conjunction with fire walls to provide fire protection between different zones of a structure, and particularly to isolate high fire risk areas of a building from the remainder of the structure, such as the garage of a dwelling from its living quarters. Fire doors usually are not capable of indefinitely withstanding the high temperature conditions of a fire but, rather, are designed to maintain the integrity of the firewall for a limited time to permit the occupants of a building to escape and to delay the spread of fire until fire control equipment can be brought to the scene.
Various tests have been designed for fire doors and are based on factors, such as the time that a given door would withstand a certain temperature while maintaining its integrity, and hose stream tests which involve the door's ability to withstand the forces of a high pressure water stream. The American Society for Testing Materials (ASTM) has devised tests to establish fire door standards and these standards are incorporated into building codes and architectural specifications. One such standard, ASTM Method E 152, requires a door to maintain its integrity for period ranging up to 1.5 hours while withstanding progressively higher temperatures and erosive effects of a high pressure stream of water from a fire hose at the conclusion of the heat (fire) exposure.
Considerations in fire door design, in addition to retarding the advance of fire, include the cost of raw materials and the cost of fabrication. Furthermore, the weight of the door is important, both from the standpoint of ease of handling and cost of transportation. The strength of the door is also an important factor, since fire doors may be required to pass the above-described water stream test as well as have the requisite strength to withstand normal use and abuse.
Fire-resistant doors have been made using a variety of constructions and utilizing a number of different materials, including wood, metal, and mineral materials. Early forms of fire doors simply comprised wooden cores faced with metal sheeting. Although wood of ample thickness is an effective fire and heat retardant, doors of such construction tend to be heavy and are expensive to fabricate and transport.
Some fire-resistant cores are constructed using such materials as expanded perlite (which functions as a lightweight inorganic filler), gypsum (which functions as a fire resistant material bonding the perlite in an integral structure), cement (which functions as a further fire resistant material and counteracts shrinkage of the core), an aqueous solution of polyvinyl alcohol (which acts as an organic binder and increases the viscosity of the mixture of ingredients while also hydrating the gypsum) and fiberglass (which functions as a reinforcing material). See for example U.S. Pat. No. 4,159,302.
According to U.S. Pat. No. 4,159,302 a fire resistant door core can be prepared by hydrating a composition containing about 50-70 percent by weight expanded perlite, about 10-30 percent by weight of calcined gypsum, 10-20 percent by weight of an hydraulic cement, such as Portland cement and 1-5 percent by weight of an organic binder, such as polyvinyl alcohol. The composition to be hydrated can also optionally contain up to 1 percent by weight of a fibrous reinforcement, up to 4 percent by weight clay and up to 4 percent by weight of unexpanded vermiculite. Cores made with this composition are disclosed as having a density between about 22 to about 30 pounds per cubic foot (pcf).
U.S. Pat. No. 5,798,010 (and related U.S. Pat. Nos. 5,155,959; 5,171,366; 5,305,577; 5,945,208 and 6,299,970) describes a fire resistant building composition useful in connection with the manufacture of fire door components. In its broadest aspects, the building material is made from about 53-78% by weight calcium sulfate and 7 to 30% by weight paper fiber and optionally other performance boosting additives including inorganic fiber reinforcement and binder polymer. An aqueous mixture of ingredients is dewatered, pressed, typically at about 300 psi, and dried to produce product having a density between 40 and 75 pounds per cubic feet (pcf) useful as fire door core components.
Other fire doors have included conventional gypsum wallboard panels as a core material. However, in order to produce sufficient fire resistance, the thickness required of the wallboard is such as to result usually in an excessively heavy door. Furthermore, internal structural members such as rails or mullions have been found necessary to support and strengthen wallboard panels. The need for such reinforcing elements increases the cost of materials and assembly of such doors. In addition to the above-mentioned considerations, fire doors must, in order to be commercially acceptable, also have other properties that are related to the manufacture, installation and service of the fire door.